The present invention relates to an SiGeC semiconductor crystal applicable to a bipolar transistor or a field-effect transistor and a method for producing the same.
The present invention relates to an SiGeC semiconductor crystal, which is a Group IV mixed crystal semiconductor, and a method for producing the same.
Conventionally, attempts have been made at fabricating a semiconductor device which operates faster than known Si semiconductor devices by stacking a Si layer and a semiconductor layer containing Si as a main ingredient thereof so as to form a heterojunction. Si1xe2x88x92xGex and Si1xe2x88x92xxe2x88x92yGexCy, which are mixed crystal semiconductors each formed using a Group IV element that is in the same group as Si, are expected as candidates for a material for forming a heterojunction with the Si layer. Particularly, as for an Si1xe2x88x92xxe2x88x92yGexCy mixed crystal semiconductor that is formed from three different elements, its band gap and lattice constant can be independently controlled by adjusting its composition, resulting in greater flexibility in semiconductor device design. Therefore, the Si1xe2x88x92xxe2x88x92yGexCy mixed crystal semiconductor has attracted much attention. For example, a lattice matching between Si1xe2x88x92xxe2x88x92yGexCy and Si crystals can be made by properly adjusting the composition of Si1xe2x88x92xxe2x88x92yGexCy. A heterobarrier (band offset) can be also formed on both a conduction band edge and a valence band edge around the interface of the heterojunction between the Si and Si1xe2x88x92xxe2x88x92yGexCy layers by properly adjusting the composition of Si1xe2x88x92xxe2x88x92yGexCy. Japanese Unexamined Patent Publication No. 10-116919, for example, discloses a field-effect transistor in which a two-dimensional electron gas serves as a carrier and which can operate at a high speed by utilizing a heterobarrier formed on the conduction band edge near the interface of Si/SiGeC layers.
Meanwhile, for producing Si1xe2x88x92xxe2x88x92yGexCy mixed crystals, use is now made of, for example, a chemical vapor deposition (CVD) process in which respective source gases of elements Si, Ge and C are dissolved so as to induce epitaxial growth of those elements on the Si or SiGe layers, or a molecular beam epitaxy (MBE) process in which respective source solids of the elements are heated and vaporized so as to induce crystal growth of the elements. In order to use an Si1xe2x88x92xxe2x88x92yGexCy layer as a part of a semiconductor device, the Si1xe2x88x92xxe2x88x92yGexCy layer is required to be doped with an impurity for generating a carrier, which will be a dopant so as to control the conductivity and specific resistance of the Si1xe2x88x92xxe2x88x92yGexCy layer. In the Si1xe2x88x92xxe2x88x92yGexCy layer, boron (B) and phosphorus (P) are used as a p-type dopant and an n-type dopant, respectively, in many cases. It is well known that the conductive type and specific resistance of a growth layer can be adjusted by doping the layer with a dopant during crystal growth.
Problems to be solved
FIG. 4 is a graph indicating the result of an experiment conducted by the inventors for the purpose of consideration as to doping of an Si1xe2x88x92xxe2x88x92yGexCy layer and shows how the specific resistance of the Si1xe2x88x92xxe2x88x92yGexCy layer changed depending on the C content thereof. The Si1xe2x88x92xxe2x88x92yGexCy layer as a sample from which the data was collected is as-grown one obtained by being epitaxially grown by a CVD process with the use of Si2H6, GeH4 and SiH3CH3 as respective source gases of elements of Si, Ge, and C and B2H6 as a source gas of boron (B) which is a p-type impurity (dopant) (i.e., through in-situ doping). In this experiment, the flow rates of Si2H6 and GeH4 and the temperature of the Si1xe2x88x92xxe2x88x92yGexCy layer during the epitaxial growth thereof were kept constant and only the flow rate of SiH3CH3 was changed. As shown in FIG. 4, as for the sample having a C content of 0.45% or less, even when the C content was changed, the specific resistance of the sample stayed almost constant and relatively low. In contrast, as for the Si1xe2x88x92xxe2x88x92yGexCy layer having a C content of 1.6%, the specific resistance thereof remarkably increased. That is to say, it was clearly shown that clearly shown that the specific resistance of the Si1xe2x88x92xxe2x88x92yGexCy layer which had been epitaxially grown by this method increased to the level at which the layer would be no longer suitable for use as an active region of a semiconductor device (e.g., a channel region of FET, a base layer of a bipolar transistor).
FIG. 5 is a graph indicating the result of the secondary ion mass spectroscopy on a sample formed basically in the same method as the sample from which the data shown in FIG. 4 was collected and shows how the boron concentration of the Si1xe2x88x92xxe2x88x92yGexCy layer changed depending on the C content thereof. This is an experiment that was conducted to examine whether the specific resistance shown in FIG. 4 was affected by the boron concentration, because the doping efficiency of boron slightly changes, depending upon the C content of the Si1xe2x88x92xxe2x88x92yGexCy layer, when boron is introduced into the Si1xe2x88x92xxe2x88x92yGexCy layer by an in-situ doping process. Note that the sample from which the data of FIG. 5 was collected is not identical to the sample from which the date of FIG. 4 was collected. As shown in FIG. 5, the B concentration of the Si1xe2x88x92xxe2x88x92yGexCy layer did not largely depend on the C content thereof. In addition, as also shown in FIG. 5, the B concentration of the Si1xe2x88x92xxe2x88x92yGexCy layer tended to increase as the C content of the Si1xe2x88x92xxe2x88x92yGexCy layer increased. That is to say, it was confirmed that the increase in the specific resistance of the sample having a B concentration of 1.6% shown in FIG. 4 was not caused due to the shortage of B concentration.
The inventors then assumed that an increase in specific resistance of regions having a relatively high C content in the Si1xe2x88x92xxe2x88x92yGexCy layer would be caused by B having not sufficiently been activated during an epitaxial growth associated with an in-situ doping. Conventionally, with an in-situ doping of a dopant during an epitaxial growth of a semiconductor layer (e.g., a Si layer or an Si1xe2x88x92xxe2x88x92yGexCy layer) using a CVD process, an annealing process for activating the dopant is considered as unnecessary because the dopant is activated concurrently with the epitaxial growth of the semiconductor layer, unlike an impurity doping by an ion implanting process. As shown in FIG. 4, the Si1xe2x88x92xxe2x88x92yGexCy layer, with its C content of 0.45% or less, has a relatively low specific resistance in the as-grown state. In such a case the Si1xe2x88x92xxe2x88x92yGexCy layer as-grown can be therefore used for an active region of a semiconductor device. However, it is likely that when the C content of the Si1xe2x88x92xxe2x88x92yGexCy layer increases, some phenomenon causing problems that cannot be solved by the conventional technology will appear. Particularly, it is empirically known that various properties of the layer are largely changed when the C content of the Si1xe2x88x92xxe2x88x92yGexCy layer increases to over 1%. Therefore, around 1% of C content can be considered to be the critical value where the specific resistance of the Si1xe2x88x92xxe2x88x92yGexCy layer starts increasing, although the data of FIG. 4 is not enough to confirm that.
It is an object of the present invention to provide an Si1xe2x88x92xxe2x88x92yGexCy semiconductor crystal applicable as an active region of a semiconductor device and a method for producing the same by taking measures to activate boron (B), particularly for an Si1xe2x88x92xxe2x88x92yGexCy layer having a relatively high degree of carbon (C) content which goes just over 1%.
A method for producing an SiGeC semiconductor crystal according to the present invention includes the steps of: a) epitaxially growing an SiGeC semiconductor crystal on a substrate, the SiGeC semiconductor crystal doped with a carrier generating impurity on a substrate the SiGeC semiconductor having a composition represented by Si1xe2x88x92xxe2x88x92yGexCy (where 0 less than x less than 1, 0.01xe2x89xa6y less than 1); and b) performing an annealing process to activate the carrier generating impurity in the SiGeC semiconductor crystal.
In this method, it was empirically confirmed that the specific resistance of an SiGeC layer could be reduced. Conventionally, it has been considered that an in-situ doped impurity is activated during an epitaxial growth. However, this idea is not applicable to the SiGeC layer and thus it is assumed that even the in-situ doped impurity can be activated enough by an annealing process.
The temperature in the annealing process is within the range from 700xc2x0 C. to 1000xc2x0 C., both inclusive, and thereby the impurity can be particularly effectively activated.
If the epitaxially growing step includes a CVD process using, as a source, a hydride which is made of at least one material selected from the group consisting of Si, Ge, C and B, the present invention is of great significance.
A SiGeC semiconductor crystal of the present invention includes two or more alternately stacked sets of: an Si1xe2x88x92zGez (where 0 less than z less than 1) layer which contains a carrier generating impurity; and an Si1xe2x88x92wCw (where 0.01xe2x89xa6w less than 1) layer which has a carrier generating impurity higher in concentration than the Si1xe2x88x92zGez layer, and functions as an SiGeC semiconductor crystal which has a composition represented by Si1xe2x88x92xxe2x88x92yGexCy (where 0 less than x less than 1, 0.01xe2x89xa6y less than 1).
Thus, the Si1xe2x88x92zGez layer and the Si1xe2x88x92wCw layer are unitized to function as a single SiGeC semiconductor crystal. At the same time, in the Si1xe2x88x92zGez layer, by utilizing activation of the carrier generating impurity as-grown without any special process performed, the SiGeC semiconductor crystal with a relatively low specific resistance which is suitable for an active region of a semiconductor device can be achieved.
If the Si1xe2x88x92zGez layer and the Si1xe2x88x92wCw layer are each smaller in thickness than the case where a discrete quantum state is generated in the layers, they can more reliably be unitized to function as a single SiGeC semiconductor crystal.
Specifically, each of the Si1xe2x88x92zGez layer and the Si1xe2x88x92wCw layer preferably has a thickness of 1.0 nm or less.